0. Uncertainty in the Base Flood Elevation – Design Considerations Texas Society of Professional Engineers – Dallas Chapter May 30, 2017

1. Introduction
Alan Johnson, P.E., CFM
Mr. Johnson has over 35 years of experience in floodplain mapping, management and insurance rating, either with FEMA, or as a FEMA contractor. From , he was lead subject matter expert for the floodplain management reviews of Hurricane Katrina/Rita or Sandy Public Assistance recovery reconstruction. Prior FEMA service includes Contracting Officer’s Technical Representative overseeing floodplain mapping and modifications west of the Mississippi River in the FEMA HQ Flood Insurance and Mitigation Administration, plus shorter assignments as a Mitigation Grants reviewer in Florida, and Individual Assistance Applicants’ evaluator in Alabama. In 2015, he rejoined FEMA, as team member in the Region 6 Mitigation staff. He holds both a B.S. and a M.S. in Civil Engineering from South Dakota St U., and U. of Nebraska, respectively.

2. INTRODUCTION WHAT GOES INTO DEVELOPING BASE FLOOD ELEVATIONS
FEMA – LOCATIONS OF INFORMATION
DESIGN CONSIDERATIONS

3. Flooding is:
Legally defined (by the National Flood Insurance Program) as
A general and temporary condition where two or more acres of normally dry land or two or more properties are inundated by water or mudflow.
Base Flood Elevation (BFE) – The elevation of flooding with a 1% chance of occurring in any given year. AKA – 100-year flood, will only be at or exceeded once in 100-years, on average.
Forms of flooding
Coastal Storm Surge
Riverine
Shallow sheet flooding
Ponding
Alluvial Fan

4. Over the past few years we have gotten our share of rain, haven’t we?
Throughout the Region we have received historic rainfalls in Oklahoma, Texas, Arkansas and Louisiana over the past 24 months.
In the years prior, the State of New Mexico was ravaged by storms bringing rainfall to areas previously ravaged by wildfire, it has been a wet couple of years for Region 6
All of us in the room know that….
Having a Flood Insurance Rate Map (FIRM) doesn’t increase or decrease your likelihood of flooding… the rain is coming, with or without a map.
Swollen rivers and water logged soils exacerbate large rain events by washing all rainfall into the streams and waterways, once the banks are overtopped, the water travels over roadway crossings, along and over streets and throughout the communities low lying areas.
These flood events do not only occur along the streams that FEMA has identified as flood prone on their Flood Insurance Rate Maps (FIRMs).
The flooding pictures on this slide are from the May 2016 flooding throughout the state, there are pictures like this throughout the state, throughout the Region
These storms expand our knowledge of flooding potential, challenge our current data, and provide a great opportunity to discuss risk and support community identified and supported mitigation activities to reduce future risks

5. What’s Needed to Establish the Base Flood Elevation?
Topography
Horizontal
Vertical
Changes over time?
Hydrology
Statistical Analyses of Gages
Rainfall/Runoff Modelling
Regression Equations
Rational Method
Hydraulics
Physical Modelling
Computer Modelling
Man-made features

6. Topography How do we know that we are at the right point?
How do we know that we have captured the correct elevations?
Topography, especially of rivers, change over time. How do we capture changes between snapshots of the floodplain?

7. High Resolution Ground Elevation Data
Models are only as good as the data they are built with
Cross-sections are collected against ground elevation data
Red depicts current data collection accuracy
White dots are survey grade ground info
FEMA Region 6 understands the necessity and requirement for building models on a great base data.
TERRAIN is most one of the most critical datasets to our NEW AND IMPROVED process.
LiDAR data is collected across River basins throughout the State over the past couple of years.
Region 6 has expended 30-50% of previous years budgets to collect this critical data element.
Models are only as good as the information you build them on.
The picture at the top shows you a digital terrain model that is built from the data collection – it provides an understanding of WHERE THE GROUND IS.
We use this information to prepare automated modeling to describe the rivers and streams throughout the Region.
In the profile view below, you can see the 30M (YELLOW) and 10M (GREEN) DEM profiles... This data was readily available for the preparation of mapping during Map Modernization
In contrast review the RED line… this is the cross-section pulled from the high resolution ground elevation.
In review of this data against some actual survey (see the white dots) the LiDAR creates a great starting point for us to prepare modeling with.

8. Hydrology
Is the hydrology today this same as yesterday? 10 or 20 years ago? 100 or 300 years?
Looking at “rare” events – highest in 100 years or 500 years. If only have 30, 50, 100 years of data, what’s the likelihood that we’ve seen a 1% or 0.2% annual chance flood?
How do you establish discharges or flood heights between measuring points (gages)?

9. Background
Interagency Science Team
InFRM Watershed Hydrology Assessments
Methods and Importance

10. Background National Flood Insurance Program (NFIP)
Flood Insurance Studies circa 1970’s - present
Dated hydrology
Regional Regression Equations common Statistical Hydrology methods
Standard methods for river hydraulics
Mapping or inundation limits

11. What Are Watershed Hydrology Assessments About
Watershed Hydrology/Meteorology
How much water
River Hydraulics
What depth will the water reach
Outcomes or Impacts

12. InFRM Science Team Hydrologic Methods
Statistical hydrology
Regression equations
Meteorological information
NWS TP40, NOAA Atlas 14, Dr. Asquith TxDOT circa 95
Rainfall runoff modeling
System simulations (period of record simulations)
Allows development of regulated and unregulated flows
Stochastic techniques

13. Use of Statistical Hydrology to Inform Flood Risk
Less costly
Rapid results
Significant influence from:
Development of storage
Non-homogeneity of watershed
Climate variability
May require record length significantly longer than the frequency or return interval to be estimated, did the water receive any significant events during the POR
Requires greater expertise than is commonly understood ;

14. 17B Statistical Analysis 5,000 600 1,600 900 5,500 4,000 200 3,100
5, ,600
,500
4, ,100
17B
Frequency
Flow (cfs)
5 Year
3,500
10 Year
6,000
100 Year
15,000
500 Year
35,000

15. 17 C B Statistical Analysis 5,000 600 1,600 2,900 6,500 5,500 5,350
5, , ,900
6, , ,350
4, , , ,500 17 C B
Frequency
Flow (cfs)
5 Year
4,500
10 Year
12,250
100 Year
32,000
500 Year
72,000

16. San Marcos River Basin Statistical Hydrology
Wimberley Gage
Present

17. 100-Year Flow Estimates - Statistical - Wimberley
154k CFS
95k CFS

18. 100-Year Depth Estimates - Statistical - Wimberley

19. Results to Be Provided Various frequency peak discharges
Trinity River Basin
Various frequency peak discharges
2-yr, 5-yr, 10-yr, 25-yr, 50-yr, 100-yr, 250-yr, 500-yr
Existing conditions
Future conditions
Climate change impacts

20. Hydraulics Engineering calculations of a natural process –
Manning’s Equation – Since 1889
Uniform flow
Function of:
Channel slope
Velocity
Flow Area
“n” values – Chow 1959
Assumptions
Fixed Bed/sides
Open flows (no blockage)
1-D modelling
Many different models, providing different results from same input data
2-D modelling
Most Coastal now 2-D, just getting cost-effective in Riverine, mainly with Time as 2nd D

21. Hydraulics Issues of Non-Uniformity
Areas with Super to Sub-critical flows (Hydraulic Jumps)
Braided Channels
Multiple bridge openings

22. Hydraulics Biggest issues in modeling – Who manages? Water crossings
Matching old/new models and data
Who manages?
COMMUNITY! (Floodplain Administrator)
FEMA (through oversight of data, and community implementation and enforcement)

23. FLOODPLAIN ADMINISTRATION
After a study is drafted, it goes through a public review and acceptance process.
Once this process is done, that floodplain is the community’s responsibility to manage, and keep the same, or inform FEMA as to how it has changed, due to development.
This is the Flood INSURANCE RATE (FIRM) – How does the insurance rates change with flood risk?

24. Flood Risk and Flood Insurance
Elevation Lowers Premiums
1 Foot Below BFE
Premium at BFE
3 Feet Above BFE
$4,786/year
$47,860/10 years
$2,136/year
$21,360/10 years
$591/year
$5,910/10 years
Assumes AE zone, amount of coverage $200/80
Rates are as of April 1, 2016
Homes built below BFE could be hit hard by an increase to full-risk rates
Elevating 3 feet above the BFE could lower premiums significantly!

25. Flood Insurance Rate Table for Zone AE (post-FIRM)
Note that rates do not go above +4 feet above BFE.
Severe cost below BFE for new or substantially improved or damaged structures because non-compliant construction.
Non-SFHA standard rates still more expensive that +1 foot rates, and about 3x more expensive than AE +3 or +4 feet.
+3 or +4’ in AE zone is about ¼ cost of standard B, C, or X rate

26. FEMA Building Science
Supports, and actively participated in developing
INTERNATIONAL BUILDING CODES
RESIDENTIAL
COMMERCIAL
ASCE-24

27. WEBSITES FOR FEMA Information

28. FLOOD MAPPING INFORMATION
MAP SERVICE CENTER
STUDIES IN PROGRESS
AVAILABLE ENGINEERING DATA AND REVISION FORMS
ESTIMATED BASE FLOOD ELEVATIONS VIEWER

29. FEMA Building Science Main Site
Publications of Flood and Wind Resiliency

30. FLOOD MAP REVISIONS 2 TYPES
NO CHANGE TO BASE FLOOD ELEVATIONS (BFE)
CHANGES BASE FLOOD ELEVATIONS
ALL REQUIREMENTS ARE LISTED AT 44 CFR 65 (OR 70)
NO CHANGE TO BFE – COMPARES BFE TO LOWEST ADJACENT GROUND OR FLOOR
INADVERTENT INCLUSION (LOMA) – PART 70
REVISION DUE TO FILL (LOMR-F) – PART 65.5
CHANGE TO BFE
REVISION DUE TO NEW HYDROLOGY/HYDRAULIC (LOMR OR PMR) – PART 65.6
USE EFFECTIVE MODELS, AND UPDATE – TIE-IN WITHIN 0.5’ ON BFE, 0.1’ ON FLOODWAY
REVISION DUE TO LEVEES OR FLOODWALLS (BARRIERS TO FLOODING) – PART 65.10

31. FLOOD MAP REVISIONS
ALL FLOOD MAP REVISIONS MUST BE COORDINATED WITH COMMUNITY FPA
COMMUNITIES ARE REQUIRED TO NOTIFY FEMA WITHIN 6 MONTHS OF MODIFICATION OF FLOODPLAIN DUE TO DEVELOPMENT (65.3)
FLOODWAY REVISIONS – PART 65.7
PROPOSED CHANGES – PART 65.8, FEES COVERED IN PART 72
MT-1 FORM FOR (C)LOMA OR LOMR-F
MT-2 FORM FOR (C)LOMR OR PMR

32. DESIGN CONSIDERATIONS
GO HIGH, IF CAN, +3’ TO BFE GETS REALLY CHEAP INSURANCE.
FLOODPROOFED/BARRIER SYSTEMS CAN BE OVERTOPPED, USUALLY WITH WORSE LOSSES THAN IF SYSTEM WASN’T THERE.
COORDINATE AND PERMIT VIA THE LOCAL FPA
ASSURE DEVELOPMENT WILL NOT INCREASE FLOWS VS NON-DEVELOPED.
IF CAN DEVELOP SITE WITHOUT CHANGING AREA IN FLOODPLAIN, DO SO.
AVOID FILLING FLOODPLAIN AS REMOVES STORAGE, AND CAN INCREASE FLOOD HEIGHTS OR FREQUENCY OF FLOODING.

33. The Vision for Our Communities

34. Questions? If you have additional questions after this presentation, please feel free to reach out to me at or or (cell).

35. Thank you!